Capture build_nexe verbose log messages to emit in the event of error.
Capture build_nexe verbose log messages to emit in the event of error.

Record log messages in memory for build_nexe runs, so that they can be
emitted for failures. The buildbots do not run with verbosity turned up
due to space / master performance constraints.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3934
BUG=https://code.google.com/p/chromium/issues/detail?id=390764
TEST=local
R=jam@chromium.org, ncbray@chromium.org, mseaborn@chromium.org

Review URL: https://codereview.chromium.org/666073003

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@13977 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client